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Lights are light scene graph locations with a light material assigned. The light material contains a shader, which defines how that light illuminates the scene.
One strength of Katana is its ability to only load parts of the scene graph at render time if they are needed. Lights can potentially be anywhere within the scene graph hierarchy. Katana needs to keep a list of all lights so it doesn’t need to traverse what could potentially be a very complicated scene graph, just to find all the lights. The light list is stored in the lightList attribute under the /root/world location.